If someone had Hepatitis C and tested 40 years after conyraction, can they get a false negative antibody test from taking azathioprine and prednisone?

Hep C. If you had hepatitis C for 40 years, azathioprine and prednisone would not be able to mask its presence. Testing for hep C can be via the virus or the antibody. See http://www.webmd.com/hepatitis/hepc-guide/hepatitis-c-exams-and-tests for more information.